1. Technical Field
The present invention relates in general to an improved data communication and networking system, and in particular to an improved local area network (LAN) switch. Still more particularly, the present invention relates to an improved LAN switch having means for switching modes of operation in response to a rate at which frames having an error pass through such a LAN switch.
2. Description of the Related Art
LAN switches are known in the art of data communication and networking, and are used to provide data communication between LAN segments connected to multiple ports of the LAN switch. A "LAN segment" may be defined as a group of nodes where all nodes utilize the same Open Systems Interconnection (OSI) model physical layer.
To connect two LAN segments, the LAN switch enables a node in one LAN segment to communicate with a node in a different LAN segment. Typically, the LAN switch receives data from a node in one LAN segment and passes such data to another LAN segment which contains a destination node. To conserve data bandwidth in all LAN segments connected to the LAN switch, the LAN switch may only transfer data from one LAN segment to another LAN segment when a node in one LAN segment attempts to communicate with a node in a second LAN segment. Moreover, such a LAN switch may transfer data only to the LAN segment containing the destination node. Thus, communication between nodes in the same LAN segment may not be transferred to another LAN segment by the LAN switch, while information communicated between nodes belonging to different LAN segments is transferred from a source LAN segment to a destination LAN segment.
Some known LAN switches are able to learn the addresses of all nodes connected to each LAN segment connected to each port of the LAN switch. After learning the addresses of LAN nodes connected to each LAN segment, the LAN switch is able to selectively transfer data from one LAN segment to another LAN segment by analyzing the source address and destination address of the communication.
Known LAN switches provide two different modes of transferring data from one LAN segment to another LAN segment. In a first mode, the LAN switch passes a data frame through the switch without temporarily storing the entire frame within the switch. This mode of operation may be referred to as a "cut-through" mode. In the cut-through mode, the LAN switch analyzes the address information contained in a frame header, and immediately begins transferring data received from one LAN segment to the destination LAN segment as determined by such address information. The problem with the cut-through mode of operation is that frames containing erroneous data are transferred from one LAN segment to another LAN segment, and upon reception by a node in the destination LAN segment, the transfer frame is then discarded. By transferring a frame containing erroneous data, bandwidth has been wasted in the LAN switch, the destination LAN segment, and the data processing system connected to the destination node.
One partial solution to the problem of transferring frames having erroneous data is to operate the LAN switch in a second mode, which is called a "store-and-forward" mode. In the store-and-forward mode, each data frame is fully received and stored or buffered in the LAN switch. Then the frame is checked for errors before the data frame is forwarded to the destination LAN segment. If the data frame contains an error, the data frame is not transferred to the destination LAN segment. The disadvantage of the store-and-forward mode of operation is that this mode increases the delay from the time a source node transmits the data to the time the data is received by a destination node. This delay is caused by storing an entire frame within the LAN switch before transferring the frame to the destination LAN segment.
Data errors in a frame of data may occur for several reasons. For example, an electrical disturbance, which may be caused by an electric motor, may cause a data bit error in the data frame. Another cause of errors in a data frame is improperly installed network wiring. Yet another cause of erroneous data is a faulty workstation connected to a node in LAN segment. In many instances, sources of interference with network data are temporary. Because measures utilized to make network data more immune to interference usually degrades the performance of the network, it is desirable to implement such measures only while the source of interference is present.
For example, a LAN switch performs most efficiently in the cut-through mode when frames of data are error free. This is because network bandwidth in the LAN switch and destination LAN segment are not unnecessarily consumed by transferring frames having errors. Moreover, the delay caused by the LAN switch is minimized in the cut-through mode. However, if the LAN switch receives a frame to be transferred to a destination LAN, and such a frame has an error, the network system may operate most efficiently if the LAN switch is in the store-and-forward mode, where the LAN switch does not transfer a frame having an error to the destination LAN segment. By not transferring a frame having an error, the LAN switch conserves data bandwidth on the output port connected to the destination LAN segment, and conserves bandwidth in the destination LAN segment. However, when the LAN switch operates in the store-and-forward mode, the transfer of the frame from the source LAN segment to the destination LAN segment is delayed due to storing an entire frame and checking the frame for an error before transferring the frame to the destination LAN segment. Thus, the implementation of the store-and-forward degrades network efficiency to some extent in order to prevent a greater degradation in efficiency caused by transferring erroneous frames and consuming data bandwidth unnecessarily in the network system.